linux-kernel - Re: [PATCH 16/19] KVM: x86: Explicitly track all possibilities for APIC map's logical modes

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Message-ID: <Yw+Sz+5rB+QNP2Z9@google.com>
Date:   Wed, 31 Aug 2022 16:56:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 16/19] KVM: x86: Explicitly track all possibilities for
 APIC map's logical modes

On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 8209caffe3ab..3b6ef36b3963 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -168,7 +168,12 @@ static bool kvm_use_posted_timer_interrupt(struct kvm_vcpu *vcpu)
> >  
> >  static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
> >  		u32 dest_id, struct kvm_lapic ***cluster, u16 *mask) {
> > -	switch (map->mode) {
> > +	switch (map->logical_mode) {
> > +	case KVM_APIC_MODE_SW_DISABLED:
> > +		/* Arbitrarily use the flat map so that @cluster isn't NULL. */
> > +		*cluster = map->xapic_flat_map;
> > +		*mask = 0;
> > +		return true;
> Could you explain why this is needed? I probably missed something.

If all vCPUs leave their APIC software disabled, or leave LDR=0, then the overall
mode will be KVM_APIC_MODE_SW_DISABLED.  In this case, the effective "mask" is '0'
because there are no targets.  And this returns %true because there are no targets,
i.e. there's no need to go down the slow path after kvm_apic_map_get_dest_lapic().

> > @@ -993,7 +1011,7 @@ static bool kvm_apic_is_broadcast_dest(struct kvm *kvm, struct kvm_lapic **src,
> >  {
> >  	if (kvm->arch.x2apic_broadcast_quirk_disabled) {
> >  		if ((irq->dest_id == APIC_BROADCAST &&
> > -				map->mode != KVM_APIC_MODE_X2APIC))
> > +		     map->logical_mode != KVM_APIC_MODE_X2APIC))
> >  			return true;
> >  		if (irq->dest_id == X2APIC_BROADCAST)
> >  			return true;
> 
> To be honest I would put that patch first, and then do all the other patches,
> this way you would avoid all of the hacks they do and removed here.

I did it this way so that I could test this patch for correctness.  Without the
bug fixes in place it's not really possible to verify this patch is 100% correct.

I completely agree that it would be a lot easier to read/understand/review if
this came first, but I'd rather not sacrifice the ability to easily test this patch.